The planned dam and power-generating station will generate electricity from the Lower Churchill Falls region, and will send power to Nova Scotia as well as Newfoundland and Labrador.
Multiple sources say the event, in the Labrador community of Happy Valley-Goose Bay, has been pulled together over the last 24 hours — a reflection of how quickly the loan negotiations have moved.
Sources told CBC News that negotiations on the loan guarantee for the proposed $7.4 billion megaproject are still happening, but progress has accelerated over the course of the week and a conclusion seems imminent.
Not only is the prime minister travelling to Goose Bay for the announcement, but sources said Newfoundland and Labrador Premier Kathy Dunderdale has made last-minute plans to go, as has Nova Scotia Premier Darrel Dexter.
Dunderdale seemed surprised earlier this afternoon when word broke of the prime minister's visit.
"Look, all I can tell you is I don't know that there's an announcement to be made in Happy-Valley Goose Bay tomorrow. I don't know that."
The federal loan guarantee is the last major piece missing in the premier's Muskrat Falls puzzle.
She has said all along that once the financing terms were in place, her cabinet would move quickly to officially sanction the project, an outcome that has seemed inevitable for quite some time.
Harper's visit also comes at a time when Peter Penashue — the lone Conservative cabinet minister in Newfoundland and Labrador — is under fire for spending thousands of dollars over the legal limit during last year's federal election.
A report issued yesterday also raised questions about Penashue's family and business ties to the Muskrat Falls project.